1. Field of the Invention
The present invention relates to a polishing apparatus and a polishing method, and more particularly to a polishing apparatus and a polishing method for polishing a substrate, such as a wafer.
2. Description of the Related Art
With a recent trend toward higher integration and higher density in semiconductor devices, circuit interconnects become finer and finer and the number of levels in multilayer interconnect is increasing. In the fabrication process of the multilayer interconnect with finer circuit, as the number of interconnect levels increases, film coverage (or step coverage) of step geometry is lowered in thin film formation because surface steps grow while following surface irregularities on a lower layer. Therefore, in order to fabricate the multilayer interconnect, it is necessary to improve the step coverage and planarize the surface. It is also necessary to planarize semiconductor device surfaces so that irregularity steps formed thereon fall within a depth of focus in optical lithography. This is because finer optical lithography entails shallower depth of focus.
Accordingly, the planarization of the semiconductor device surfaces is becoming more important in the fabrication process of the semiconductor devices. Chemical mechanical polishing (CMP) is the most important technique in the surface planarization. This chemical mechanical polishing is a process of polishing a wafer by placing the wafer in sliding contact with a polishing surface of a polishing pad while supplying a polishing liquid containing abrasive grains, such as silica (SiO2), onto the polishing surface.
A polishing apparatus for performing CMP has a polishing table that supports the polishing pad having the polishing surface, and a substrate holder for holding the wafer. The substrate holder is often called a top ring or a polishing head. This polishing apparatus polishes the wafer as follows. The top ring holds the wafer and presses it against the polishing surface of the polishing pad at predetermined pressure. The polishing table and the top ring are moved relative to each other to bring the wafer into sliding contact with the polishing surface to thereby polish a surface of the wafer.
When polishing the wafer, if a relative pressing force applied between the wafer and the polishing pad is not uniform over the surface of the wafer in its entirety, insufficient polishing or excessive polishing would occur depending on the pressing force applied to each portion of the wafer. Thus, in order to even the pressing force exerted on the wafer, the top ring has at its lower part a pressure chamber formed by a flexible membrane (or a membrane). This pressure chamber is supplied with fluid, such as air, to press the wafer against the polishing surface of the polishing pad through the membrane under the fluid pressure.
The above-described polishing pad has elasticity. As a result, the pressing force becomes non-uniform in an edge portion of the wafer during polishing of the wafer. Such non-uniform pressing force would result in so-called “rounded edge” which is excessive polishing that occurs only in the edge portion of the wafer. In order to prevent such rounded edge, the top ring has a retaining ring for retaining the edge portion of the wafer. This retaining ring is configured to be vertically movable relative to a top ring body (or carrier head body) and press a portion of the polishing surface of the polishing pad around the wafer.
A substrate transfer device, which is called a pusher, is disposed near the top ring. This pusher is configured to elevate the wafer, which has been transported by a transporter (e.g., a transfer robot), and transport the wafer to the top ring that has been moved to a position above the pusher. The pusher further has a function to receive the wafer from the top ring and transport the wafer to the transporter (e.g., the transfer robot). The pusher has a substrate detection sensor for detecting the presence of the wafer. When the substrate detection sensor detects the wafer, the pusher performs its next operations.
In the above-discussed polishing apparatus, the wafer, which has been polished on the polishing surface of the polishing pad, is attracted to the top ring via vacuum suction. The top ring is elevated and then the wafer is released from the top ring. This wafer releasing operation is performed by supplying the fluid into the pressure chamber to inflate the membrane so as to separate the wafer from the membrane.
However, since the wafer is separated as a result of deformation of the membrane, a central portion of the wafer may fail to be separated from a corresponding central portion of the membrane which is not deformed greatly. Thus, in order to assist in releasing the wafer from the top ring, a release nozzle (i.e., a substrate separation assisting device) is provided near the pusher. This release nozzle ejects a jet of fluid (release shower) toward a contact portion between the wafer and the membrane to thereby assist the wafer release. Hereinafter, releasing of the wafer in the conventional technique will be described with reference to FIG. 11 and FIG. 12.
FIG. 11 is a schematic view showing a wafer releasing operation when releasing the wafer from the membrane. As shown in FIG. 11, a membrane 104 is attached to a lower surface of a top ring 100. When a wafer W is transported, the wafer W is held via vacuum suction on a substrate holding surface 104a which is constituted by the membrane 104. FIG. 11 shows a state in which the membrane 104 is inflated so as to release the wafer W therefrom.
A release nozzle 153 for ejecting a jet of a fluid 160 is provided neat the top ring 100. Specifically, the release nozzle 153 is located so as to eject the fluid 160 onto a contact portion between the wafer W and the membrane 104. The fluid 160 may be a fluid mixture of pure water and N2 (nitrogen). When the jet of the fluid 160 impinges on the contact portion between the wafer W and the membrane 104, the wafer W is easily released from the top ring 100.
A substrate detection sensor 170 for detecting the presence of the wafer W is mounted to an upper surface of a pusher 150. When the wafer W is released from the top ring 100 and the wafer W is detected by the substrate detection sensor 170, the pusher 150 is lowered so that the transporter (e.g., the transfer robot) can remove the wafer W from the pusher 150.
FIG. 12 is a schematic view showing a state in which the membrane 104 is stretched. In recent years, the wafer has been becoming heavier with an increase in its diameter, while there is a tendency to use a soft material for the membrane in order to reduce a load on the wafer. Accordingly, when the pressure is supplied into the membrane 104 to inflate it so as to separate the wafer W from the membrane 104, if the central portion of the wafer W remains sticking to the membrane 104, then the membrane 104 is stretched due to the own weight of the wafer W and the inflated membrane 104.
When the membrane 104 is stretched greatly, the contact portion between the wafer W and the membrane 104 is lowered by T1 from a position before the membrane 104 is stretched (this position is indicated by a two-dot chain line). Since the fluid 160 is ejected toward the position that is set before the membrane 104 is stretched, the fluid 160 does not impinge on the contact portion between the wafer W and the membrane 104 (i.e., the position lowered by T1). As a result, it is not possible to quickly release the wafer W.
Moreover, if the membrane 104 is stretched greatly, the substrate detection sensor 170 mounted to the pusher 150 may sense the wafer W that is still sticking to the membrane 104, i.e., may incorrectly detect that releasing of the wafer W is completed. As a result of such a false detection, despite the fact that the wafer W is not released from the membrane 104, the pusher 150 may perform the next operation, causing damage to the wafer W. Therefore, there is a need to improve the process of releasing the wafer W from the membrane 104.
CMP is a process which is likely to produce scrap wafers. There are cases where more than half of the scrap wafers that have been produced in the semiconductor device fabrication come from CMP. In particular, when the wafer is attracted to the top ring by the vacuum suction for a long time and the membrane is then inflated so as to separate the wafer from the membrane, a local stress may be applied to the wafer. As a result, fine interconnects formed on the wafer may be fractured or the wafer itself may be damaged. Accordingly, improving the wafer release process leads to an improvement of productivity.